The invention relates to composite pigments comprising carbonate crystals precipitated in the presence of and chemically bonded to a pigment substrate. The invention, in especially preferred embodiment comprises novel hybrid white pigments in which individual anionically charged silicate mineral pigment particles, preferably hydrous kaolin, serve as a substrate for crystals of an alkaline earth metal carbonate precipitated on the surface of the anionically charged mineral pigment particles and adherent thereto.
White pigments such as hydrous and calcined kaolin, chemically bulked kaolin, precipitated and naturally occurring calcium carbonate, titania, mica, talc, alumina and silica are widely used to coat or fill paper goods, in paints, plastics, rubber goods and the like. Much research and development effort has been expended to enhance the performance properties of these pigments. In some cases, this has involved blending pigments to achieve benefits not realized using single pigments.
An obvious extension to the efforts provide such pigments has been to affix particles of one class of pigments to particles of a different class of pigments in preference to using xe2x80x9cloose blendsxe2x80x9d. As an example, coflocculation has been advocated. Another example is described in WO 97/32934.
Other efforts have been directed to surface coating minerals particles with inorganic gels or mixed gels.
For many years, specially processed kaolin clays were minerals the most widely used by the paper industry, one of the largest consumers of white pigments. The kaolin, in hydrous and calcined form, was used to coat paper (and board) as well as to fill paper goods. In recent years, precipitated calcium carbonate (PCC) has replaced kaolin in some of those applications, especially paper filling, in which neutral or alkaline conditions can be employed. In the past, paper filling was conducted exclusively at acidic conditions that were feasible using clay. However, recently, PCC is finding increasing use by the paper industry. Nevertheless, under acid papermaking conditions, the use of PCC is still under scrutiny owing to the decomposition of PCC to calcium ions and carbon dioxide gas. But, PCC has the advantages of forming different crystal forms to achieve a variety of end properties on paper.
The present invention provide novel composite inorganic pigment comprising (a) a substrate composed of individual particles of a silicate and/or oxide mineral and (b) an adherent surface coating comprising a multiplicity of crystals of a metal carbonate precipitated in-situ and chemically bonded to surfaces of the substance. The present invention also provides a method for making such pigments involving pH control and the initial addition of the source of (b) in the presence of water to (a) in a manner such that the source of (b) is adsorbed on the surface of (a). Thereafter, precipitation of the adsorbed source of ions is carried out by pH adjustment. The sequence of adsorption and precipitation may be repeated one or more times.
In especially preferred embodiment, a alkaline source of calcium ions is adsorbed on the surfaces of the negatively charged platy particles of kaolin clay (hydrous and calcined) clay and pH is decreased by adding carbon dioxide to the resulting aqueous slurry of kaolin with adsorbed calcium ions. A family of hybrid pigments containing both a negatively charged component (kaolin) and positively component (PCC) is obtained. Since the amount of calcium that can be adsorbed is limited by the tendency of polyvalent cations to flocculate and thicken the clay slurry, in a presently preferred embodiment, the sequence is usually repeated one or more times to provide an adequate coating of PCC crystals. It is noted that the PCC content far exceeds that achievable when the quantity of calcium introduced is limited to that obtained when relying on ion exchange.
An unexpected benefit observed when conducting experimental tests directed to making kaolin/PCC hybrids was that the processing desirably limited the content of ultrafines and slimes in the feed kaolin without necessitating physical removal of slimes. In other words, the ultrafines are physically present but particle size distribution is engineered to contain less fines by enlargement as a result of practicing the process of the subject invention technology. It is well known that removal of fines or slimes from the kaolin pigments is beneficial and, in some cases, necessary to achieve optimum optical and Theological properties of the kaolin based fillers and coating products.
Several prior art references disclose bulking or chemical structuring of kaolin particles where the fines and slimes are preferentially bound with the help of binding agents, usually a cationic polyelectrolyte, without removing them. However, a surprising and salient aspect of the present technology is the ability to engineer the particle size distribution of the end product without actually removing the fines and ultrafines. For example, the Electron micrographs shown in FIGS. 1 and 2 illustrate the particle size enlargement of NUCLAY(copyright) kaolin particles by practice of this invention.
The extent of precipitation can be such that it varies from partial coverage of the kaolin surface (in patches) to complete encapsulation of calcium carbonate on kaolin, depending upon the desired particle size distribution and optical properties.
The invention in preferred embodiment provides value added kaolin based composite pigments of kaolin with precipitated calcium carbonate (PCC) that exhibit unique pigment characteristics and in some cases, a synergistic advantage over loose blend of kaolin and PCC in end applications such as coating and filling paper.
In the present invention, the calcium carbonate phase is grown in-situ on the kaolin surface for engineering particle size distribution and managing surface morphology. The resulting composite kaolin calcium carbonate pigment surprisingly showed extremely low amount of fines indicating strong bulking tendencies of fines of the kaolin substrate.